Fix dart2js warnings and add test to ensure it stays clean.

pkg/compiler/lib/src/inferrer/inferrer_engine.dart

Index: pkg/compiler/lib/src/inferrer/inferrer_engine.dart
diff --git a/pkg/compiler/lib/src/inferrer/inferrer_engine.dart b/pkg/compiler/lib/src/inferrer/inferrer_engine.dart
index e91a884b004a2f3b14c173578f1e5ccac30bca8e..3296ff1481fcc9c3e716ea3934accb2ce40cb797 100644
--- a/pkg/compiler/lib/src/inferrer/inferrer_engine.dart
+++ b/pkg/compiler/lib/src/inferrer/inferrer_engine.dart
@@ -474,11 +474,12 @@ class InferrerEngine {
     if (analyzedElements.contains(element)) return;
     analyzedElements.add(element);
 
-    var visitor = compiler.options.kernelGlobalInference
+    dynamic visitor = compiler.options.kernelGlobalInference
         ? new KernelTypeGraphBuilder(element, resolvedAst, compiler, this)
         : new ElementGraphBuilder(element, resolvedAst, compiler, this);
     TypeInformation type;
     reporter.withCurrentElement(element, () {
+      // ignore: UNDEFINED_METHOD
       type = visitor.run();
     });
     addedInGraph++;